REAPER Plug-in Extensions are additional components which can add functionality to REAPER. These plug-ins are not plug-ins in the traditional "DAW" sense of plug-ins, in that they are not "Effect" plug-ins, but rather can do many of the following things:

REAPER itself can generate the latest extension API header file, to ensure that development is in sync with a particular REAPER release version. The header will be written to the REAPER program directory (therefore you may need to run REAPER as admin, depending on your disk permissions).From REAPER, run this action:

[developer] Write C++ API functions header

The very popular SWS extension is an open source project, and contains many excellent examples of how to do useful things with the REAPER Extension SDK.